1. Field of the Invention
This invention relates to digital transmission systems and, more particularly, to automatic equalizer circuits for differing lengths of transmission cable in such systems.
2. Description of the Prior Art
In digital transmission systems using telephone cables, pulses of substantially uniform amplitude and duration are transmitted between terminal facilities separated by various lengths of twisted pairs of conductors. At the terminals, the receiver must distinguish, during each digit period, between the presence and absence of a pulse and, for each pulse detected, regenerate a new pulse for decoding.
Due to the frequency-dependent nature of the twisted pair, it is necessary to provide equalization for the frequency distortion which takes place in the transmission cable. Moreoever, since the attenuation characteristic of a twisted pair is a function of cable length, equalization must be adjustable for differing cable lengths. As a result, it is normally necessary to employ a variable line equalizer network which, when connected in tandem with the section of telephone cable, compensates for the frequency-dependent characteristic of the connected cable section.
It can be seen that the provision of a line equalizer network is an essential part of the design requirement for a digital transmission system. Moreover, these line equalizer networks must be designed so as to provide the appropriate gain characteristics to compensate for the attenuation of a wide variety of different cable lengths and gauges. This requirement would normally require the stocking of a large number of different networks, measuring the characteristics of the cable section in question and selecting and adjusting a network to provide the desired compensation. This procedure would require a large inventory of networks as well as manual procedures for measuring cable sections and for adjusting the network.
It is known in the prior art to provide a line equalizer which automatically adapts to the particular length and gauge of cable to which it is connected. One such network is disclosed in R. A. Tarbox U.S. Pat. No. 3,568,100, granted Mar. 2, 1971. The Tarbox equalizer utilizes an electronically controlled gain circuit responsive to the characteristics of the line to adjust the frequency shaping of the gain so as to provide automatic equalization for different lengths and gauges of cable. The Tarbox circuit arrangement used a variable impedance network having a single pole which is movable in the frequency domain in response to a derived control signal. This arrangement provides an approximately correct high frequency compensation for most lengths and gauges of cable in the telephone plant and is suitable for high frequency pulse transmission systems such as the T-1 carrier system operating at 1.544 megabits.
The Tarbox arrangement provides a linear low frequency gain characteristic and adjusts the shape of the high frequency gain characteristic in response to the automatic measurement. The assumption that the low frequency characteristic is linear, however, provides totally inadequate equalization for systems in which the low frequency components of the pulse signals are of significant magnitude since such sections of cable do, in fact, have nonlinear low frequency characteristics.